Light modulation elements are used for window panes of buildings and vehicles, interior materials, and the like. Particularly in recent years, demand and expectation for light modulation elements have been increased from the viewpoints of reducing a cooling and heating load, reducing a lighting load, improving comfort, and so on.
As the light modulation element, the following systems have been developed: an electric field drive system in which a liquid crystal material or an electrochromic material is used, and the light transmittance is controlled by application of an electric field; a thermochromic system using a thermochromic material, of which the light transmittance is changed with a temperature; a gas chromic system in which the light transmittance is controlled by controlling an atmospheric gas.
Examples of the method for controlling the light transmittance include a method of switching between transmission and scattering of light by a light modulation material, a method of switching between transmission and absorption of light, and a method of switching between transmission and reflection of light. Among them, a hydrogen-activation-type light modulation element that switches between transmission and reflection of light by hydrogenation and dehydrogenation of a light modulation material has an advantage that light from outside can be reflected to prevent inflow of heat, and therefore excellent heat shielding property is exhibited, so that a high energy saving effect is obtained. In addition, since it is able to switch between hydrogenation and dehydrogenation by a gas chromic system, it is possible to increase the area and reduce the cost.
As hydrogen-activation-type light modulation materials capable of reversibly switching between a transparent state and a reflective state by hydrogenation and dehydrogenation, rare earth metals such as yttrium, lanthanum and gadolinium, alloys of a rare earth metal and magnesium, alloys of an alkaline earth metals such as calcium, strontium or barium and magnesium, and alloys of a transition metal such as nickel, manganese, cobalt or iron and magnesium are known. Particularly, when a magnesium alloy is used as the light modulation material, a light modulation element having a high light transmittance in a transparent state is obtained because magnesium hydride has a visible light transmittance.
In the hydrogen-activation-type light modulation element, a catalyst layer is provided in proximity to a light modulation layer composed of a light modulation material. The catalyst layer has a function of promoting hydrogenation and dehydrogenation of the light modulation layer. As the catalyst layer, palladium, platinum, a palladium alloy, a platinum alloy or the like is used. As such a hydrogen-activation-type light modulation element, one including a light modulation layer and a catalyst layer on a glass substrate has been heretofore studied (e.g., Patent Document 1). For the hydrogen-activation-type light modulation material, it is known that when switching between a transparent state by hydrogenation and a reflection state by dehydrogenation is repeated, switching characteristics may be deteriorated because magnesium in a magnesium alloy in a light modulation layer passes through a catalyst layer, and is deposited on the surface, and oxidized. It has been suggested that a buffer layer such as a metal thin-film or a hydrogenated metal thin-film is provided between a light modulation layer and a catalyst layer for suppressing migration of magnesium to the catalyst layer (e.g., Patent Document 2).